Recently developed ionic devices rely on the movement of ions in ionic conductors to change electrical or other properties of the devices. For example, FIG. 1A shows an ionic device 100, which includes a layer or film 120 of an ionic conductor that is sandwiched between two electrodes 110 and 130. Ionic conductor 120 may be a layer of titanium dioxide (TiO2), while ions 125 are oxygen vacancies, i.e., gaps in the crystal structure where oxygen is missing. With titanium dioxide and oxygen vacancies, ionic device 100 can behave as a memristor because a voltage difference applied between electrodes 110 and 130 can drive ion currents that move oxygen vacancies and significantly alter the electrical resistance ionic conductor 120. For a display device, ionic conductor 120 can be a layer of tungsten trioxide (WO3), while ions 125 are lithium ions which are sufficiently mobile in tungsten trioxide to move in response to an applied voltage. Pure tungsten trioxide is clear, but lithium impurities give tungsten trioxide a blue color. Accordingly, ion currents that move lithium ions to or from a display surface can change the color of the surface of ionic device 100.
FIG. 1A shows a configuration of device 100 in which ions 125 are concentrated near one electrode 110. Layer 120 may initially be fabricated in this configuration by forming two layers 122 and 124 with distinct compositions, e.g., one layer 122 containing a primary material such as titanium dioxide TiO2 and the other layer 124 containing a source material such as oxygen-depleted titanium dioxide The source material is the initial source of the mobile ions. Application of a voltage having the proper polarity and sufficient magnitude between electrodes 110 and 130 can then drive, an ion current that moves ions 125 from layer 122 into layer 124 to switch device 100 from the state shown in FIG. 1A in which ions are concentrated near electrode 110 to the state shown in FIG. 1B in which more ions 125 are dispersed throughout ionic conductor 120. The distribution of ions 125 in FIG. 1B can, for example, convert an insulating layer 122 of pure titanium dioxide to a semiconductor layer resulting when titanium dioxide is doped with oxygen vacancies. Continued application of a high voltage of the same polarity can switch device 100 to the state of FIG. 1C where the ions are highly concentrated near electrode 120. Device 100 can similarly switch back from the state of FIG. 1B or 1C to the state of FIG. 1A by application of an opposite polarity voltage of sufficient magnitude to drive an ion current that moves ions 125 toward electrode 110. Switching between the states of FIGS. 1A and 1B is particularly useful for ionic memristive devices, while switching between the states of FIGS. 1A and 1C may be useful for ionic display devices. These operations are possible because ionic conductor 120 provides sufficient mobility for movement of ions 125 of a species that is capable of significantly altering the properties of ionic conductor 120 or device 100 as a whole.
Non-volatile operation of ionic devices such as device 100 is often desired. For example, for use as a non-volatile memristive memory cell, device 100 might have a high voltage applied with a polarity selected to switch device 100 to the high resistance state corresponding to FIG. 1A or 1C or a low resistance state corresponding to FIG. 1B in order to write a binary value 0 or 1 to device 100. A lower voltage that causes an electron current but minimal ion movement can then be used to detect or measure the resistance of device 100 and read the value previously written. However, higher mobilities of ions in ionic conductor 120, which are desirable for fast switching, permit movement of ions when a read voltage is applied for a read operation and even when no external voltage is applied. Typically, an ionic device has only one stable ionic concentration profile (e.g., uniformly distributed ions as in FIG. 1B) corresponding to the thermodynamic equilibrium and an ionic device tends to relax, e.g. by diffusion, toward the stable concentration profile. The rate at which an ionic device will, relax can be significant. For example, drift-diffusion, which controls the relaxation time, may be just V times slower than the ion current during switching, where V is the applied switching voltage in units of thermal voltage VT=kBT/e where kB is the Boltzmann constant, e is the electron charge and T is the temperature. For typical voltages used for the thin film ionic devices, the ratio of relaxation time to switching time may only be a few thousands, so that fast switching devices may have poor non-volatile retention. In many applications, both fast switching and long retention times are desired.
Use of the same reference symbols in different figures indicates, similar or identical items.